In the future, soft, animal-inspired robots may be safely deployed in difficult-to-access environments in which rigid robots cannot currently be used such as inside the human body or in spaces that are too dangerous for humans to work. Centimeter-sized soft robots have been created, but thus far it has not been possible to fabricate multifunctional flexible robots that can move and operate at smaller size scales.
An integrated fabrication process was developed that enables the design of soft robots on the millimeter scale with micrometer-scale features. The robotic soft spider was created from a single elastic material with body-shaping, motion, and color features.
While the smallest soft robotic systems are very simple — usually with only one degree of freedom — the new hybrid technology merges three different fabrication techniques to create the robotic spider made only of silicone rubber with 18 degrees of freedom, encompassing changes in structure, motion, and color, with tiny features in the micrometer range. The technology could pave the way towards achieving similar levels of complexity and functionality on this small scale as those exhibited by their rigid counterparts.
A soft lithography technique was used to generate 12 layers of an elastic silicone that together constitute the soft spider’s material basis. Each layer is precisely cut out of a mold with a laser-micromachining technique, and then bonded to the one below to create the rough 3D structure of the soft spider.
Key to transforming this intermediate structure into the final design is a preconceived network of hollow microfluidic channels integrated into individual layers. With a third technique — injection-induced self-folding — one set of these integrated microfluidic channels was pressurized with a curable resin from the outside. This induces individual layers — and with them, their neighboring layers — to locally bend into their final configuration, which is fixed in space when the resin hardens. This way, for example, the soft spider’s swollen abdomen and downward-curved legs become permanent features.
The origami-like folding process can be controlled by varying the thickness and relative consistency of the silicone material adjacent to the channels across different layers, or by laser-cutting at different distances from the channels. During pressurization, the channels then function as actuators that induce a permanent structural change. The remaining set of integrated microfluidic channels was used as additional actuators to colorize the eyes and simulate the abdominal color patterns of the peacock spider species by flowing colored fluids, and to induce walking-like movements in the leg structures. The system was fabricated in a single, monolithic process that can be performed in a few days and easily iterated in design optimization efforts.
The small size and flexibility of these robots could enable a new approach to endoscopy and microsurgery.